Enhanced energy recovery from cassava ethanol wastewater through sequential dark hydrogen, photo hydrogen and methane fermentation combined with ammonium removal

Cassava ethanol wastewater (CEW) was subjected to sequential dark H₂, photo H₂ and CH₄ fermentation to maximize H₂ production and energy yield. A relatively low H₂ yield of 23.6 mL/g soluble chemical oxygen demand (CODs) was obtained in dark fermentation. To eliminate the inhibition of excessive NH₄⁺ on sequential photo fermentation, zeolite was used to remove NH₄⁺ in residual dark solution (86.5% removal efficiency). The treated solution from 5 g CODs/L of CEW achieved the highest photo H₂ yield of 369.7 mL/g CODs, while the solution from 20 g CODs/L gave the lowest yield of 259.6 mL/g CODs. This can be explained that photo H₂ yield was correlated to soluble metabolic products (SMPs) yield in dark fermentation, and specific SMPs yield decreased from 38.0 to 18.1 mM/g CODs. The total energy yield significantly increased to 8.39 kJ/g CODs by combining methanogenesis with a CH₄ yield of 117.9 mL/g CODs.